In a coating apparatus of the curtain coating type, a moving support is coated by causing a free falling curtain of coating liquid, referred to hereafter as simply the curtain, to impinge on the moving support to form a layer thereon. An apparatus to perform this method is described in U.S. Pat. No. 3,508,947 to Hughes wherein a multilayer composite of a plurality of distinct layers is formed on a slide hopper and dropped therefrom to form a falling curtain.
In the curtain coating process, particularly as used to manufacture multi-layer photographic materials, the quality of the coating is largely determined by the properties of the liquid curtain. It is important to insure that a stable laminar flow of coating solution is formed by the slide hopper and that an equally stable laminar liquid curtain is formed from that coating solution. To prevent contraction of the edges of the falling curtain under the effect of surface tension it is known that the curtain must be guided at its edges by curtain edge guides.
It is well known in the curtain coating art that introduction of a lubricating liquid between the curtain and the edge guide will improve the operation of the curtain. These improvements include the ability to maintain the curtain at lower total flow rates with lubricating liquid than without, and the ability to maintain curtains of higher viscosity with a lubricating liquid than without. Typically, the lubricating liquid is simply water, however, an alternate liquid of low viscosity may be used for the same purpose.
The momentum of the solutions at the coating point is a critical variable in determining the size of the window of operability of the curtain coating process. If the momentum is low, the maximum coating speed attainable before the onset of air entrainment is reduced. Therefore, for an internal edging process (coating within the edges of a web), the lubricating liquid must be introduced as close to the hopper lip as possible to maximize the momentum of the solution near the edge of the curtain at the coating point. This is to minimize the span the curtain must travel with a non-lubricated wall at the edge. Any velocity which is lost due to wall drag at the edges, with respect to the velocity of the curtain sufficiently far from the edge guides to be unaffected by the velocity drag of the edge guides, cannot be regained. Hence, at the coating point, the edges of the curtain will have lower momentum than will the middle due to wall drag along the edge guide. This results in a smaller window of operability at the edges of the curtain than in the middle. This limits the maximum speed attainable for the entire curtain. This coating speed reduction due to momentum loss at the edges can have a severe negative impact on the efficiency of a manufacturing operation employing curtain coating.
The prior art does not address a significant problem that can occur during the introduction of this lubricating liquid. This is turbulent flow from the outlet for the lubricating liquid at the top of the edge guide. If the flow is turbulent at this point the resulting edge will be wavy, meaning the coating width will randomly change due to the chaotic nature of the flow of the lubricating liquid. Edge waviness reduces the overall quality of the coating as well as increasing the potential for waste in manufacturing. Turbulent flow of the lubricating liquid can also produce waves in the curtain, which propagate from the edge into the main body of the curtain, and which can form streak imperfections in the coating where they meet the substrate.
In laminar flow, turbulent flow is initiated at disturbances and will decay to fully laminar flow according to empirical relationships. A sharp corner, a rough wall, an abrupt change in geometry and many other disturbances will initiate turbulent flow. Turbulent and laminar flow regimes are generally classified through use of the Reynolds number, Re. This is a dimensionless group of parameters used to relate the inertial forces in a flow to the viscous forces. At high Reynolds numbers turbulence is more likely than at low Reynolds numbers. For different flow geometries, experiments have determined Reynolds number ranges which classify the laminar flow, transition regions and fully turbulent flow region. It is therefore desirable to be operating in the laminar flow region for the specific geometry being used. However, in the laminar flow region disturbances may still initiate turbulence but these disturbances will then decay. The rate of decay is dependent upon the magnitude of the Reynolds number, the lower the Reynolds number, the quicker disturbances will decay. The rate of decay, or length that the flow must continue past the disturbance to be free of turbulence can be estimated by calculating the entry length, L.sub.e. The entry length is a measure of how much distance the liquid must travel after a disturbance, for example, the inlet of a channel, to form a fully developed laminar flow profile. For tube flow (circular cross-section) this is the distance after the inlet into the tube it takes to develop Poiseuille flow.
The present invention describes an apparatus and method for optimizing the geometry of the lubricating fluid delivery tube or channel to allow for the outlet to be placed very close to the hopper lip, while avoiding turbulence at the outlet. This results in being able to coat at higher speeds due to an increase in momentum at the edges of the curtain and the elimination of wavy edges and curtain waves due to turbulent flow of the lubricating liquid.